1. Field of the Invention
The present invention relates to a reflectometry and a reflectometer which measure the loss distribution of an optical waveguide on the basis of Rayleigh back-scattering profile, and more particularly to a method and apparatus implementing spatial resolution of a millimeter region by using a so-called optical low coherence reflectometer utilizing optical low coherence interference.
2. Description of Related Art
FIG. 1 shows a typical return waveform obtained by measuring Rayleigh back-scattering from a 70 km long optical fiber cable by an OTDR (Optical Time Domain Reflectometer). The attenuation of the signal along the length of the optical fiber gives an attenuation coefficient (dB/km) of the optical fiber, and the amount of a stepwise attenuation of the signal gives splice loss at a splice point. The OTDR, however, cannot be used for measuring a waveguide, because its length is several meters at most. Instead, a so-called optical low coherence reflectometer is used which utilizes the interference of low-coherent light.
FIG. 2 shows a basic setup of a conventional optical low coherence reflectometer (OLCR). In this figure, the reference numeral 1 designates a low-coherent light source, which suppresses laser oscillation to emit low-coherent light whose spectral width is, for example, 20 nm. The reference numeral 2 designates an optical fiber coupler, 3 and 4 designate branch ports of the coupler 2, 5 designates a collimator lens, 6 designates a mirror, 7 designates a waveguide under test, 8 designates an output port of the coupler 2, 9 designates a photodetector, and 10 designates a selective level meter.
The emitted light from the light source 1 is divided into two parts by the coupler 2. The first part is passed through the port 3, collimated by the collimator lens 5, reflected by the mirror 6, and incident onto the coupler 2 again, so that the light propagating the output port 8 is used as local oscillator light. The other part is passed through the port 4, and incident onto the waveguide 7 under test. Back-scattered light, which is produced by Rayleigh back-scattering throughout the length of the waveguide 7 under test, travels through the port 4, and is combined with the local oscillator light. The combined light is received by the photodetector 9, and the interference intensity is measured by the selective level meter 10.
When performing measurement by the setup, the mirror 6 is shifted step by step by a very small amount, and the interference intensity for respective positions of the mirror 6 is detected by the selective level meter 10. Since the coherent length of the light source 1 is approximately 20 .mu.m, the reflected light (reverse scattered light) interferes with the local oscillator light only when the optical path length difference between the reflected light and the local oscillator light is within 20 .mu.m. Accordingly, each position of the mirror 6 can be considered to have a one-to-one correspondence with each one of the back-scattering positions along the waveguide 7, and the interference intensity measured at each point is proportional to the reflection intensity. This is the principle of the conventional setup, which makes it possible to measure the back-scattering from the waveguide 7 by shifting the mirror 6.
FIG. 3 shows a setup of the low-coherent light source 1 using an erbium-doped optical fiber. In this figure, the reference numeral 11 designates a pump source module whose emission wavelength is 1.48 .mu.m. The reference numeral 12 designates a WDM (Wavelength-Division Multiplexer) coupler, 13 designates an erbium-doped optical fiber, 14 designates a mirror, and 15 designates an optical isolator which is incorporated to suppress reflection. The light emitted from the pump source module 11 enters the erbium-doped optical fiber 13. Then, the optical fiber 13 emits superfluorescent light whose center wavelength is about 1550 nm and whose spectral width is 20 nm by launching the pump light into the fiber. The superfluorescent light is reflected by the mirror 14, and is incident onto the optical fiber 13, again. The incident light is amplified by the optical fiber 13, propagates straight through the WDM coupler 12, and passes through the optical isolator 15. This setup makes it possible to suppress the laser oscillation by the optical isolator 15, and to emit the amplified superfluorescent light, thereby implementing a low-coherent light source whose spectral width is 20 nm and whose optical output is 1 mW.
This type of an optical low coherence reflectometer measures the reflection power using the interference between the oscillator light and the reflected light. Since scattering particles are distributed throughout the waveguide 7 under test, the detection signal depends not only on the reflection intensity of the individual scattering particles but also on the optical path differences between scattering particles, that is, the phase differences between scattered light. Therefore, the Rayleigh scattering particles, whose distribution varies randomly along the length of the waveguide with correlation lengths of submicron order, cause speckle noise which varies randomly at a pitch of several tens of microns along the length of the wavelength as shown in FIG. 4.
In the experiment of FIG. 4, the mirror 6 of FIG. 2 was shifted 70 times at every 15 .mu.m interval, and the reflection light from the waveguide 7 under test (a silica-based optical waveguide) was detected at each position of the mirror 6. This means that the waveguide 7 under test was detected at every 10 .mu.m interval, because the refractive index of the waveguide 7 is 1.5, and hence, 15 .mu.m/1.5=10 .mu.m. Since the 700 .mu.m long waveguide had little loss, it was expected that the detection signal had an approximately constant intensity over that length. Actually, however, a detection signal, which changed randomly as shown in FIG. 4, was observed.
One of the methods for obtaining loss information from the signal which changes randomly as in FIG. 4 is a smoothing method which averages the signal along the length of the waveguide. The smoothing method is described in K. Takada, et al. "Jagged appearance of Rayleigh back-scattered signal in ultrahigh-resolution optical time-domain reflectometry based on low-coherence interference", Optics Letters, Vol. 16, No. 18, pp. 1433-1435, Sep. 15, 1991.
This method will be explained more specifically referring to FIGS. 5, and 5A in which a 40 cm long silica-based waveguide is tested. In this method, the waveguide under test is divided into N=400 intervals in 1 mm increments, for example, and an averaged Rayleigh back-scattered signal is obtained in each interval. To perform this, the mirror 6 is shifted 100 times in 15 .mu.m steps in each interval of 1 mm as shown in FIG. 5A, and the signal is detected at each position of the mirror. After completing the total of 100 measurements in an interval, the average of these signals is calculated. The average is adopted as the Rayleigh back-scattered signal of that interval. Thus, this measurement requires 100 times intermittent shifts of the mirror 6 and the detections of the reflected signals to obtain the average value at each interval. This means that 40,000 (=100.times.400) times of shifts of the mirror 6 and detections of the reflected signals are necessary to test the entire length of the 40 cm long waveguide 7 under test. In other words, the conventional method requires a great number of shifts of the mirror 6 and detections of the reflected signals in order to pick up the loss information of the waveguide. This arises a problem in that the measurement takes a long time, and mechanical abrasion of a translation stage, on which the mirror is mounted, due to the shift of the mirror is large.
In addition, since the mirror vibrates slightly after the 15 .mu.m step shift is completed, the detection of the reflected signal should be carried out after the vibration falls. This requires approximately 0.1 second, and hence, the measurement at a particular position of the mirror requires at least 0.1 second, which in turn means that a minimum of 4,000 seconds (=0.1.times.40,000) is necessary to complete the test of the 40 cm long waveguide. Furthermore, to reduce the detection time to less than 0.1 second, the response time must be increased by widening the detection bandwidth. This causes a new problem in that the signal-to-noise ratio (S/N) of the detection is degraded and its sensitivity is reduced. In a common experiment, since the bandwidth of the detection is set at about 3 Hz to increase its sensitivity, the measurement takes about 1 second for each position.